6.9: Acid-Base Titration: Unknown HCl Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acid-base titration, this is done by delivering a titrant of known concentration into an analyte of known volume. In this assignment, you will titrate a 0.2564 M solution of sodium hydroxide, NaOH, into 25.00 mL of an unknown concentration of hydrochloric acid, HCl, and calculate the concentration of the HCl solution. - Titrant: solution in the burette, NaOH, the known Analyte: solution in the beaker, HCl, the unknown When an acid and a base react, they neutralize each other. The balanced equation for the reaction of HCl and NaOH is given below. Note the 1:1 mole ratio between the HCl and the NaOH. HCl (aq) + NaOH (aq) Concentration (M) Volume (mL) ? 25.00 → NaOH (aq) + H2O (l) 0.2564 measured The volume of NaOH used is determined by the difference in the buret readings. It is measured in mL, converted to liters, and used to calculate the number of moles of NaOH. Then the stoichiometric ratio (1:1) is used to calculate the number of moles of the unknown HCl. Because the volume of HCl used is known, the concentration of the HCl solution can be calculated. In this experiment, you will do two trials, and report your answer as the average of the two. SUBMISSION Follow the experimental procedure and record data into the provided data table. Calculate the average concentration of HCl and record in the data table. Organize and label all calculations on a separate sheet. Upload both this worksheet and the calculations sheet as your submission. PROCEDURE It is important that you read the entire procedure before beginning. Additionally, re-read each step before acting on it. TRIAL 1 1. Start Virtual ChemLab, select Acid-Base Chemistry, and then select Acid-Base Titration: Unknown HCl from the list of assignments. The lab will open in the Titrations laboratory. Record the unknown sample number. 2. The buret is filled with 0.2564 M NaOH to the 0.00 mark. The beaker has 25.00 mL of unknown HCl. The pH meter is turned on and has been calibrated. The indicator is bromocresol green. 3. Perform the titration. Click the Save button in the Buret Zoom View window so the titration data can be saved. The horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between. Stopcock positions: Fully closed Slow drops Fast drops Slow flow Full flow 4. At first, turn the stopcock to one of the flow positions. Observe the titration curve. When the blue line in the graph window (the pH curve) and the red line (conductivity curve) just touch each other, move the stopcock to add volume drop by drop. Keep an eye on the color of the solution. As soon as the yellow color changes to blue, stop adding NaOH. At that point, the pH curve should just be making its turn up to the basic, high pH side. Record the final liquid level in the buret to 2 decimal places. WARNING: If you see that your pH curve has risen above pH of 11, or starts to curve right and level off, that means too much NaOH has been added. In that case, repeat the trial by exiting the simulation and restarting. Switch to drop by drop. Stop adding NaOH. Added too much. Repeat. TRIALS 2 and 3 1. Drag the beaker to the waste container to discard it. 2. Open the beaker drawer, take a beaker, and place it to the left of the buret stand. Then fill it halfway with the unknown HCl solution. 3. Open the pipet drawer and pick up a 25 mL pipet. Bring it above the beaker you filled with HCl. It will be automatically affixed to the stand, and a blue pipette bulb will be attached. 4. Click on the blue pipet bulb. The pipet will automatically be filled with the HCl solution. 5. Discard the beaker of HCl into waste. 6. Place a new clean beaker under the pipet and click the pipet bulb to dispense 25.00 mL of the HCl into it. 7. Place the beaker on the stir plate. A stir bar is automatically added, and the solution starts being stirred. 8. Click on the bromocresol green indicator and drag the dropper over the beaker to add to the beaker. The solution will instantly turn yellow. 9. Measure and record the NaOH level in the buret as the initial level. 10. At first, turn the stopcock to one of the flow positions. Observe the titration curve. When the blue line in the graph window (the pH curve) and the red line (conductivity curve) just touch each other, move the stopcock to add volume drop by drop. Keep an eye on the color of the solution. As soon as the yellow color changes to blue, stop adding NaOH. At that point, the pH curve should just be making its turn up to the basic, high pH side. Record the final liquid level in the buret to 2 decimal places. WARNING: If you see that your pH curve has risen above pH of 11, or starts to curve right and level off, that means too much NaOH has been added. In that case, repeat the trial by repeating trial 2. 11. Repeat steps 1 through 10 as trial 3. DATA TABLE Unknown sample number: ______ 1 Initial level of NaOH, ml Final level of NaOH, ml Volume of NaOH used, mL Volume of NaOH, L Molarity of NaOH, mol/L Number of moles of NaOH Number of moles of HCl Volume of HCl, L Concentration of HCl, mol/L Average concentration of HCl, mol/L Trial 2 3
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